In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, when a wireless device is powered on, or when it moves between cells in a cellular communications network, the wireless devices receives and synchronizes to downlink signals (i.e., signals transmitted from a network node serving the cell) in a cell search procedure. One purpose of this cell search procedure is to identify the best cell according to some quality requirement and to achieve time and frequency synchronization to the cellular communications network in the downlink.
A simplified and typical initial cell search procedure as exemplified by the Long Term evolution (LTE) Release 8 will be summarized next. A wireless device has typically a frequency error of 2 to 20 ppm (Part Per Million) at power on. This corresponds to 40 to 400 kHz frequency error at a carrier frequency of 2 GHz.
Primary Synchronization Signals (PSS) and Secondary Synchronization Signals (SSS) are by the wireless device used during the cell search procedure. In the case of frequency division duplex (FDD), the PSS is transmitted in the last orthogonal frequency-division multiplexing (OFDM) symbol of slots 0 and 10 within a frame and the SSS is transmitted in the OFDM symbol preceding PSS. In the case of time division duplex (TDD), the PSS is transmitted in the third OFDM symbol of slots 3 and 13 within a frame, and the SSS is transmitted in the last OFDM symbol of slots 2 and 12, i.e., three symbols ahead of the PSS.
The wireless device then tries to detect the PSS from which it can derive the cell ID within a cell-identity group, which consists of three different cell identities corresponding to three different PSS. During this detection, the wireless device thus has to blindly search for all of these three possible cell identities. The UE also achieves OFDM symbol synchronization and a coarse frequency offset estimation with an accuracy of about 1 kHz. The latter is estimated by the wireless device by evaluating several hypotheses of the frequency error.
The wireless device can then continue to detect the SSS from which it acquires the physical cell ID and achieves radio frame synchronization. Here, the wireless device also detects if a normal or extended cyclic prefix is used. If the wireless device is not preconfigured for one of FDD or TDD, the wireless device can detect the duplex mode (i.e., FDD or TDD) by the position in the frame of the detected SSS in relation to the detected PSS. Fine frequency offset estimation can be estimated by correlating the PSS and the SSS. Alternatively, such fine frequency offset estimation is estimated by the wireless device using Cell specific Reference Signals (CRS).
After these synchronizations, the wireless device can receive and decode a Master Information Block (MIB) which is transmitted on the Physical Broadcast Channel (PBCH).
The number of OFDM symbols used for the Physical Downlink Control Channel (PDCCH) is signaled by the Physical Control Format Indicator Channel (PCFICH). This PCFICH must thus be decoded before the wireless device can receive the PDCCH. Here, the number of OFDM symbols as signaled by PCFICH can be 1, 2 or 3 for large bandwidth allocations (more than 10 resource blocks), and 2, 3 or 4 OFDM symbols for small bandwidths (less than or equal to 10 RB). The first OFDM symbols of a sub-frame are used for PDCCH.
Additional broadcasted information is transmitted in the System-Information Blocks (SIBs) which are carried by the PDSCH. This PDSCH can by the wireless device be decoded after reading the PCFICH and the PDCCH. Here, the second SIB, denoted SIB2, includes information regarding uplink cell bandwidth and random access configurations. Thus, after successful decoding of SIB2, the wireless device can transmit a preamble on the PRACH and receive a random access response (RAR) on the PDSCH.
Formats of initial synchronization signals which utilize many time resources have been proposed for future wireless systems. In general terms, these formats are based on several transmissions of the PSS and SSS sequences. For example, the same PSS sequence may be repeated four times within a sub frame such that the receiver of the PSS can accumulate the received PSS either coherently or non-coherently. For communications networks using many antennas and which rely on beamforming for good link budget, the PSS may be beamformed with different beamformers in the different OFDM symbols.
A PRACH preamble has been suggested that is based on a short sequence of the same length as the length of the OFDM symbols used for all other physical channels. The preamble sequence is created by repeating the short sequence a number of times. A receiver structure for this preamble format uses a Fast Fourier Transform (FFTs) of the same size as for other uplink channels and signals. In the receiver for the PRACH preamble, several received signals from different FFT windows can be combined. Different combinations of these FFT windows are proposed depending of the amount of phase noise, frequency errors, and speed at which the wireless device is moving. By using this proposed preamble format, a detection of PRACH is achieved which is more robust against frequency errors as compared to the preamble format in LTE release 8.
Several broadcasted signals and channels are always transmitted in LTE release 8, where these transmissions occupy a significant fraction of the available bandwidth. With the use of repeated or beamformed synchronization signals, the overhead is further increased. These signals and channels are transmitted independent of the traffic load, or numbers of users, in LTE release 8 based communications networks.
Hence, there is still a need for an improved synchronization in a wireless communications network.